The present invention relates to apparatus for measuring the radius of curvature of curved objects such as lenses. It is applicable in particular in the optical field to contact lenses for use in ophalmology, to allow the radius of curvature of the concave face of a pliable contact lens to be measured.
Contact lenses, contact glasses, or pliable or "soft" hydrophilic lenses are intended to press directly against the cornea of the patient's eye and in each case their concave face needs to follow the shape of the cornea accurately. The radius of curvature of the concave face of each lens made should therefore be known as accurately as possible to allow the lens to be classified. A number of methods and pieces of apparatus exist at the present time to allow this radius to be measured.
A first method consists in opening up each lens sequentially to a set of spherical gauges of known curvature until the lens matches to one particular gauge i.e. the one whose convex face matches the concave face of the lens. This method has the drawback of being time-consuming to put into practice, of involving subjective factors to some degree, and of providing only a moderate standard of accuracy which cannot be closer than the difference in curvature between two gauges.
Another method employing optical technology consists in placing the lens in water, in directing a beam of light onto it, and in making measurements by analysing the light rays reflected by the appropriate face of the lens. This method is however complicated to put into practice since the reflection phenomena at this face are interferred with by extraneous reflections and refractions (refraction and reflection at the other face, interference reflection at the surface of the liquid etc). In addition, when placed in the water the lens is unstable and it is a delicate matter to make the measurements.
Another known method consists in forming an image of the lens on a screen by means of a projection device and in superimposing gauges of different radii on this image until one is found which coincides. However, although this method is capable of greater accuracy than that in which the spherical gauges are employed, it still suffers from the other drawbacks of the latter method (the subjective factor, the time taken). What is more, a lens is a three-dimensional object and it is a delicate matter to perform the focussing operations to form a sharp outline image through the transparent material.
Another known method consists in arranging a rigid lens on a support which supports it at its edge, in applying a pointed contacting plunger to the centre of the lens, in observing through a binocular viewer the moment at which the plunger makes contact with the lens, in stopping the movement of the plunger at this moment, and in measuring its travel, which allows the radius to be found. This method is not however applicable to pliable lenses. In effect, although in the case of rigid contact lenses it is possible to detect the moment at which the plunger comes into contact with the lens by detecting the moment when the lens starts to lift, it is very difficult to detect this moment accurately with pliable lenses since the lens begins to deform without the observer being aware of it. It should also be noted that, even in the case of rigid lenses, this method has the drawback of involving subjective factors to some degree since its accuracy depends upon the visual acuity of the observer and his ability to detect the lifting of the lens as soon as it starts.
Mention should also be made of U.S. Pat. No. 3,135,055, which relates to a different field but which discloses a system for checking a part of which certain means have obvious analogies with certain means of the invention. The purpose of this system is however different from that of the invention, since it consists in measuring the deformation of a rigid part from an ideal shape, and its structure makes it suited only to determining dimensional errors at various points of the rigid part, which is held immobile, in relation to nominal dimensions previously placed in store. This system is completely unsuited to providing a value for the radius of curvature of a light and possibly pliable object such as a contact lens directly in only one (or a small number of) measurement operations. In particular the system disclosed in this U.S. patent specification assumes that the part being dealt with is held completely immobile in all three dimensions during measurement and, even if the electronic means of this system were to be modified to enable a radius of curvature to be measured (which is in itself not an obvious thing to do) such a system would still not be suitable in practice for application to pliable contact lenses, which are impossible to clamp completely immobile in all dimensions in a simple and speedy fashion and without distorting them.
It is a primary object of the invention to alleviate or minimize the disadvantages of the conventional methods of measurement set forth above and to provide a measuring apparatus, which by a simple and swift operation, enables the radius of curvature of a lens to be measured with great accuracy and with no subjective factors being involved. To simply the terminology, the term "lens" will be used hereinafter, but it should be understood that this term must not be taken in a limiting sense and that the invention is applicable to measuring the radius of curvature of any other similar curved object.
Another object is, in addition, to enable the radius of curvature of pliable curved objects, such as contact lenses made of synthetic hydrophilic material, to be measured with no risk of error due to deformation of the lens or object.